This makes it very easy and convenient to simultaneously share a given book. I have searched high and low, I can't find any indication that this is any way illegal, or even contrary to Amazon's terms of service.

The only real catch that I see is, they all have to be on the same account. So some level of trust and cooperation is required, but not as much as you might think. Instead of buying the books using that account--which would require somebody's credit card to be attached to it, and thus available to all within your circle--you just take turns gifting the book of the month to the shared account. So mostly, you just have to trust that nobody in your circle will go rogue and take over the account and lock you out.

You also might think the everybody's progress would stomp on each other, but that also does not seem to be the case, in my experience (which, I admit, is limited to a 2-user-sized circle). When I open the Kindle app, it asks me if I want to synch to the furthest point read (because Mom is always ahead of me), but I just have to say no.

The only trifling issue that leaves, as far as I can see, is that everybody's highlights and annotations are going to get mixed together. For most people, I see that as a total non-issue.